The present invention relates to a magnetic recording medium, and more particularly, to a magnetic recording medium having as low a cobalt content as possible, specifically a molar ratio of cobalt to iron of not more than 0.06:1 and as high a coercive force as possible, specifically not less than 1,800 Oe, and exhibiting an excellent stability independent to change in passage of time and a large output in a high recording frequency region.
In recent years, in magnetic recording apparatuses such as a hard disc drive, there has been a remarkable tendency that information devices or systems used therewith are miniaturized and required to have a high reliability. In association with such a recent tendency, in order to deal with a large capacity data, there is an increasing demand for magnetic recording media on which information can be stored with a high density.
In order to fulfill such requirements, the magnetic recording media have been strongly required to have as high a coercive force as possible.
As magnetic recording media having a large coercive force, there are widely known those comprising a substrate and a magnetic thin film formed on the substrate.
The magnetic thin films which have been already put into practice, are generally classified into magnetic iron oxide thin films composed of maghemite, etc. (refer to "Technical Report of Electronic Telecommunication Institute", published by Electronic Telecommunication Institute, (1981) MR81-20, pp. 5 to 12), and alloy thin films composed of Co--Cr alloy or the like.
The magnetic iron oxide thin films are excellent in oxidation resistance or corrosion resistance because maghemite is already oxidized. Therefore, the magnetic iron oxide thin films can show an excellent stability independent to change in passage of time, i.e., less change in magnetic properties with passage of time. However, the coercive force of the magnetic iron oxide thin films is as low as at most about 700 Oe.
Whereas, the alloy thin films have a coercive force as high as not less than about 2,000 Oe. However, the alloy thin films tend to be readily oxidized, so that the stability independent to change in passage of time is deteriorated.
In order to prevent magnetic properties of these alloy thin films from being deteriorated due to the oxidation, the surfaces of the alloy thin films have been coated with a protective film, e.g., a carbon film having usually a thickness of about 100 to about 200 .ANG.. However, in such a case, a distance between a magnetic write head and a magnetic recording layer becomes increased by the thickness of the carbon film, resulting in large loss due to the magnetic spacing, which disables the magnetic recording medium to be applied to high-density recording.
In consequence, it has been attempted to enhance the coercive force and saturation magnetization of the above-mentioned magnetic iron oxide thin films which are excellent in oxidation resistance, corrosion resistance and stability independent to change in passage of time, by incorporating cobalt or the like thereinto. The magnetic iron oxide thin films containing cobalt, etc., have been already put into practice.
In the magnetic cobalt-containing iron oxide thin films, the more the cobalt content, the higher the coercive force thereof becomes. However, with the increase of cobalt content, the stability independent to change in passage of time of these magnetic iron oxide thin films tend to be deteriorated by adverse influences of heat or the like.
Thus, at the present time, there has been a strong demand for providing magnetic recording media for high-density recording which are suitable for use in current magnetic recording systems equipped with a ring-type magnetic write head. For this reason, various kinds of magnetic recording media mentioned above have been extensively developed. Among them, the magnetic iron oxide thin films having excellent oxidation resistance and corrosion resistance have been considered to be most useful, so that it have been more strongly demanded to improve properties of these magnetic iron oxide thin films.
That is, as described above, the magnetic iron oxide thin films have been strongly required not only to have a high coercive force even when the cobalt content therein is as low as possible, but also to exhibit a large output in a high recording frequency region.
Conventionally, as typical magnetic iron oxide thin films, there are known maghemite films. It is also known that cobalt can be incorporated into the maghemite in order to enhance the coercive force thereof (Japanese Patent Publications (KOKOKU) Nos. 51-4086(1976) and 5-63925(1993), and "CERAMICS", published by Japan Ceramics Institute (1986), Vol. 24, No. 1, pp. 21 to 24).
More specifically, in Japanese Patent Publication (KOKOKU) No. 51-4086(1976), there is described a magnetic thin film memory comprising a ceramic substrate made of a high-purity alumina ceramic material which has a high density and is free from damaging memory properties of ferrite due to the reaction therewith even at an elevated temperature of 1,000 to 1,300.degree. C. to produce the ferrite, and a cobalt-based spinel-type ferrite film formed on the substrate and having a composition represented by the general formula of Co.sub.(1-x) Fe.sub.(2+x) O.sub.4 where -0.2.ltoreq..times..ltoreq.0.3, the ferrite film being composed of fine crystalline aggregates of the ferrite.
In Japanese Patent Publication (KOKOKU) No. 5-63925(1993), there is described a process for producing a magnetic iron oxide thin film having a high coercive force, comprising reactive sputtering an iron alloy as a target in an oxidization atmosphere of argon and oxygen to directly form ferromagnetic oxide magnetite (Fe.sub.3 O.sub.4) of a non-stoichiometric composition having a resistivity of 1.times.10.sup.-1 to 8.times.10.sup.-1 .OMEGA..cm, and heat-treating the ferromagnetic oxide magnetite (Fe.sub.3 O.sub.4) having the above-mentioned resistivity in an atmosphere at a temperature of 280 to 350.degree. C., thereby producing ferromagnetic oxide .gamma.--Fe.sub.2 O.sub.3 having a high coercive force.
At the present time, as the magnetic recording media for high-density recording which are suitable for use in current magnetic recording systems using a ring-type magnetic write head, there have been demanded those having a higher coercive force, an excellent stability independent to change in passage of time, and a large output in a high recording frequency region. However, there have not been provided yet such magnetic recording media having all of the above-mentioned properties.
That is, the above-mentioned known cobalt-containing maghemite thin films are still unsatisfactory in coercive force relative to the amount of cobalt contained. When it is attempted to obtain a high coercive force, specifically not less than 1,800 Oe, it is necessary to incorporate a large amount of cobalt so that the molar ratio of Co to Fe is more than 0.06:1. In this case, such cobalt-containing maghemite thin film suffers from severe deterioration in magnetic properties with passage of time by adverse influences of heat or the like.
Also, there is a problem that the Co-containing maghemite thin film cannot show a sufficiently large output in a high recording frequency region.
As a result of the present inventors' earnest studies to solve the above problems, it has been found that by forming a Co-containing maghemite thin film on a substrate so as to control a cobalt content of the Co-containing maghemite thin film to such an amount that the molar ratio of Co to Fe is not more than 0.06:1, and to adjust a spacing of a (311) plane to not more than 2.510 .ANG., a spacing of a (222) plane to not more than 2.415 .ANG. or a spacing of a (220) plane to not more than 2.950 .ANG., the obtained magnetic recording medium has a coercive force as high as not less than 1,800 Oe, and exhibits an excellent stability independent to change in passage of time and a large output in a high recording frequency region. The present invention has been attained on the basis of the finding.